Metal high dielectric constant transistor with reverse-t gate

ABSTRACT

A transistor is provided. The transistor includes a silicon layer including a source region and a drain region. A gate stack is disposed on the silicon layer between the source region and the drain region. The gate stack comprises a first layer of a high dielectric constant material, a second layer comprising a metal or metal alloy, and a third layer comprising silicon or polysilicon. A lateral extent of the second layer of the gate stack is substantially greater than a lateral extent of the third layer of the gate stack. Also provided are methods for fabricating such a transistor.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 12/113,527, filed May 1,2008, now ______. The entire disclosure of prior application Ser. No.12/113,527 is herein incorporated by reference.

Additionally, this application is related to application “Transistorwith High-K Dielectric Sidewall Spacer,” Ser. No. ______, now ______,and application “Method for Fabricating a Metal High Dielectric ConstantTransistor with Reverse-T Gate,” Ser. No. ______, now ______, which werefiled on the same day as application Ser. No. 12/113,527 and commonlyassigned therewith to International Business Machines Corporation. Theserelated applications are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of semiconductors,and more particularly relates to metal high dielectric constanttransistors.

BACKGROUND OF THE INVENTION

Metal high dielectric constant (high-k) transistors, or “MHKtransistors”, are experiencing extremely active development in theindustry. One observed problem with such transistors relates to thepresence of an elevated outer fringe capacitance Cof, on the order of40-80 aF/μm. This elevated capacitance Cof occurs because the gatesidewall of an MHK transistor no longer depletes as in a transistor witha conventional polysilicon gate. The elevated value of outer fringecapacitance Cof is of concern because it at least impairs high frequencyoperation of the MHK transistor. The elevated value of this capacitanceCof has a performance impact of approximately 1.25% per 10 aF, resultingin a 5%-10% decrease in AC performance. Current technologies do notprovide a reduction in the parasitic Miller capacitance when metal-likematerials (such as TiN) are used.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a transistor. Thetransistor includes a silicon layer including a source region and adrain region. A gate stack is disposed on the silicon layer between thesource region and the drain region. The gate stack comprises a firstlayer of a high dielectric constant material, a second layer comprisinga metal or metal alloy, and a third layer comprising silicon orpolysilicon. A lateral extent of the second layer of the gate stack issubstantially greater than a lateral extent of the third layer of thegate stack.

Another embodiment of the present invention provides a method forfabricating a transistor. A silicon layer is provided and a first layeris formed on the silicon layer. A second layer is formed on the firstlayer, and a third layer is formed on the second layer. The first layerincludes a high dielectric constant material, the second layer includesa metal or metal alloy, and the third layer comprises silicon orpolysilicon. The first, second, and third layers are etched so as toform a gate stack. After this etching, the third layer of the gate stackis etched without etching the first and second layers of the gate stack,so as to substantially reduce the width of the third layer of the gatestack.

Yet another embodiment of the present invention provides another methodfor fabricating a transistor. A silicon layer is provided, and a firstlayer is formed on the silicon layer. A second layer is formed on thefirst layer, and a third layer is formed on the second layer. The firstlayer includes a high dielectric constant material, the second layerincludes a metal or metal alloy, and the third layer comprises siliconor polysilicon. The third layer is etched without etching the first andsecond layers. After this etching, a spacer layer is deposited andetched so as to form a spacer on the sidewalls of the third layer. Afterthis etching, the first and second layers are etched without etching thefirst layer, so as to form a gate stack comprising the first, second,and third layers. A lateral extent of the second layer of the gate stackis substantially greater than a lateral extent of the third layer of thegate stack.

Other objects, features, and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and specificexamples, while indicating preferred embodiments of the presentinvention, are given by way of illustration only and variousmodifications may naturally be performed without deviating from thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional metal high dielectricconstant transistor;

FIG. 2 is a cross-sectional view of a metal high dielectric constanttransistor having a reverse-T gate in accordance with one embodiment ofthe present invention;

FIGS. 3-7 are cross-sectional views of a process for fabricating a metalhigh dielectric constant transistor having a reverse-T gate inaccordance with a first embodiment of the present invention; and

FIGS. 8-12 are cross-sectional views of a process for fabricating ametal high dielectric constant transistor having a reverse-T gate inaccordance with a second embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide metal high dielectricconstant (high-k) transistors (“MHK transistors”) with a reverse-T gate.The reverse-T gate includes a polysilicon layer with a substantiallyreduced width, which results in an increase in the distance between thepolysilicon gate layer and the contact stud. Therefore, parasiticcapacitance between the polysilicon gate layer and the contact stud isreduced.

FIG. 1 shows a conventional MHK transistor, and FIG. 2 shows an MHKtransistor having a reverse-T gate in accordance with one embodiment ofthe present invention. With respect to the conventional MHK transistor100, a parasitic gate-to-contact capacitance is made up of a capacitance104 between the metal gate layer 106 and the contact stud 108, and acapacitance 110 between the polysilicon gate layer 112 and the contactstud 108.

The MHK transistor 200 of FIG. 2 also has such a parasitic capacitance.However, in embodiments of the present invention, the polysilicon gatelayer width is less than the width of the metal gate layer. For example,in this embodiment, the width of the polysilicon gate layer 212 isbetween about ⅓ and ½ of the width of the metal gate layer. Because thewidth of the polysilicon gate layer 212 is substantially reduced, thedistance between the polysilicon gate layer 212 and the contact stud 208is increased. Therefore, the capacitance between the polysilicon gatelayer 212 and the contact stud 208 is reduced, which results in aparasitic gate-to-contact capacitance that is lower than in theconventional MHK transistor. As pitch scaling continues, this reductionin parasitic capacitance becomes more substantial.

FIGS. 3-7 show one embodiment of a process for fabricating an MHKtransistor with a reverse-T gate. The process begins with asilicon-on-insulator (SOI) wafer that has, formed on a siliconsubstrate, an overlying oxide layer (“BOX”) 314 (e.g., of 3 μm), andoverlying silicon layer 316. A conventional high-k dielectric layer 318and a metal layer 320 are deposited on the silicon layer 316. In thisembodiment, the high-k layer 318 has an exemplary thickness in the rangeof about 1-3 nm, and comprises a material having a dielectric constant(k) in the range of about 20-25 (as compared to 3.9 for SiO₂), such ashafnium dioxide (HfO₂). The metal (or metal-like) layer 320 comprises ametal or metal alloy such as titanium nitride (TiN), and has a thicknessof about 10 nm. These two layers 318 and 320 form the (as yetunpatterned) MHK gate stack layers. This initial structure represents aconventional SOI CMOS with an MHK gate stack. A polysilicon or(amorphous silicon) layer 312 is then deposited on top of the metallayer 320, with a thickness in the range of about 30-100 nm.

FIG. 3 shows the transistor formation process after a conventional gatestack etch has been performed (without showing the underlying siliconsubstrate for simplicity). In this embodiment, the gate stack etch stopsat the silicon layer 316. After the gate stack is etched, a disposablespacer 424 is formed on the sidewalls of the gate stack, as shown inFIG. 4. The disposable spacer 424 of this embodiment is a nitride spacerthat is formed by depositing a 5-50 nm thick nitride layer (e.g., usingRTCVD or PECVD) and then performing a reactive ion etch (RIE) that stopson an underlying oxide liner so as not to consume any of the underlyingsilicon.

Photolithography and ion implantation are then used to definesource/drain extensions. For an NFET the implant is performed using ann-type species, and for a PFET the implant is performed using a p-typespecies. Thus, source/drain extensions 426 are formed.

The disposable spacer 424 that was used to offset the ion implantationfrom the gate edge is then removed, such as through a hot phosphoricacid etch, other wet dip process, or through a highly selective RIEetch. After the disposable spacer 424 is removed, the width of thepolysilicon layer 312 of the gate stack is then substantially reducedusing a process that is selective between the polysilicon and the otherexposed materials, such as RIE or wet etching. In this exemplaryembodiment, the width of the polysilicon layer 312 is reduced to betweenabout ⅓ and ½ of the width of the metal layer 320. This creates the“reverse-T” gate 202, as shown in FIG. 5. That is, a lateral extent(width) of the high-k and metal layers 318 and 320 is substantiallygreater than a lateral extent (width) of the polysilicon layer 312 ofthe gate stack. As explained above, this substantial reduction in thewidth of the polysilicon layer 312 results in a reduction in theparasitic capacitance between the polysilicon layer and the adjacentcontact stud.

As shown in FIG. 6, at least one oxide and/or nitride diffusion spacer730 is formed by depositing and etching one or more layers of nitrideand/or oxide (for example, using PECVD). The diffusion spacer 730 ofthis embodiment has an exemplary thickness of about 2-10 nm. Source anddrain regions are then implanted. The source/drain implant is performedusing a p-type species for an NFET (for example, As or P) or using ann-type species for a PFET (for example, B or BF₂). A subsequent rapidthermal anneal (RTA) is performed (e.g., millisecond laser anneal orflash anneal) to provide relatively deep diffusions for the source anddrain regions 732, which are separated by the gate region.

Subsequent conventional processing is used to silicide the gate, source,and drain (typically with Ni or Co) to complete the transistor, as shownin FIG. 7. The silicide contact areas 834 and 836 are formed using thediffusion spacer 730 for alignment. In particular, a portion for thecontact area is removed (e.g., through a wet etch using HF), a metal isdeposited, an anneal is performed to form silicide, and then the metalis selectively removed so as to leave the silicide (e.g., through anaqua regia wet etch). In this exemplary embodiment, the metal is nickel,cobalt, titanium, or platinum.

FIGS. 8-12 show another embodiment of a process for fabricating an MHKtransistor with a reverse-T gate. This fabrication process is the sameas the first embodiment through the deposition of the polysilicon (oramorphous silicon) layer of the gate stack. In particular, there is anSOI wafer that has, formed on a silicon substrate, an overlying oxidelayer (“BOX”) 914, and an overlying silicon layer 916. A high-k layer918 (e.g., of hafnium dioxide) and a metal layer 920 (e.g., of titaniumnitride) are deposited on the silicon layer 916. The polysilicon (oramorphous silicon) layer 912 is then deposited on the metal layer 920.

At this point, in the process of the second embodiment the polysiliconlayer 912 is etched using a wet etch and/or dry etch process, as shownin FIG. 8. Any etching process can be used that is selective between thepolysilicon layer 912 and the metal layer 920.

After the polysilicon is etched, a spacer 1024 is formed on thesidewalls of the polysilicon layer 912, as shown in FIG. 9. The spacer1024 of this embodiment is a nitride spacer that is formed by depositinga nitride layer (e.g., using RTCVD or PECVD) and then performing an RIEthat stops on an underlying oxide liner so as not to consume any of theunderlying silicon.

After the spacer is formed, a gate stack etch is performed to etch themetal layer 920 and the high-k layer 920 of the gate stack, as shown inFIG. 10. The spacer 1024 and the polysilicon layer 912 are used as amask for the gate etch, which stops on the silicon layer 916. In thisembodiment, a dry etch process such as a plasma etch and/or a wet etchprocess such as a chemical etch are used to etch the metal and high-klayers of the gate stack.

Thus, the fabrication process of this embodiment also creates a“reverse-T” gate, as shown in FIG. 10. That is, a lateral extent (width)of the high-k and metal layers 918 and 920 is substantially greater thana lateral extent (width) of the polysilicon layer 912 of the gate stack.For example, in this embodiment, the width of the polysilicon layer 912is between about ⅓ and ½ of the width of the metal layer 920. Asexplained above, this substantial reduction in the width of thepolysilicon layer 912 results in a reduction in the parasiticcapacitance between the polysilicon layer and the adjacent contact stud.

Next, an additional spacer 1238 is formed over the sidewalls of thespacer 1024, the metal layer 920, and the high-k dielectric layer 918,as shown in FIG. 11. This additional spacer 1238 is formed by depositingnitride and/or oxide layers and then performing an etch.

Photolithography and ion implantation are then used to definesource/drain extensions, as explained above with respect to the firstembodiment. The additional spacer 1238 is used to offset the ionimplantation from the gate edge.

As shown in FIG. 12, a diffusion spacer 1330 is formed by depositing andetching nitride and/or oxide (for example, by PECVD). The diffusionspacer 1330 of this embodiment has a thickness of about 2-10 nm. Sourceand drain regions are then implanted. The source/drain implant isperformed using a p-type species for an NFET (for example, As or P) orusing an n-type species for a PFET (for example, B or BF₂). A subsequentrapid thermal annealing RTA provides relatively deep diffusions for thesource and drain regions 1332, which are separated by the gate region.Subsequent conventional processing is used to silicide the gate, source,and drain (typically with Ni or Co) to complete the transistor, asexplained above with respect to the first embodiment.

Accordingly, the present invention provides metal high-k dielectric(MHK) transistors with a reverse-T gate. This reverse-T gate is a gatestack having a polysilicon layer with a substantially reduced width,which increases the distance between the polysilicon layer of the gatestack and the adjacent contact stud. Therefore, the parasiticcapacitance between the polysilicon layer and the contact stud isreduced.

The embodiments of the present invention described above are meant to beillustrative of the principles of the present invention. These MHKdevice fabrication processes are compatible with CMOS semiconductorfabrication methodology, and thus various modifications and adaptationscan be made by one of ordinary skill in the art. All such modificationsstill fall within the scope of the present invention.

For example, while the exemplary embodiments of the present inventiondescribed above relate to gate structures that use hafnium dioxide forthe high-k layer and titanium nitride for the metal layer, furtherembodiments can use other compatible materials, such as ZrO₂ orHfSi_(x)O_(y), which both exhibit the high dielectric constant (e.g., kof approximately 20-25) needed to provide a larger equivalent oxidethickness. Similarly, other metal oxide-based materials may be used,such as a uniform or a composite layer comprised of one or more ofTa₂O₅, TiO₂, Al₂O₃, Y₂O₃ and La₂O₅. The metal-containing layer 114 couldalso be formed of another material, such as one or more of Ta, TaN,TaCN, TaSiN, TaSi, AlN, W and Mo. Additionally, the upper layer 312 ofthe gate stack can be comprised of any material that is able to beetched, remain conductive, and withstand high temperatures. Similarly,while the embodiments described above relate to a transistor on an SOIwafer, the transistors and fabrication methods of the present inventionare also applicable to bulk technologies. Likewise, the various layerthicknesses, material types, deposition techniques, and the likediscussed above are not meant to be limiting.

Furthermore, some of the features of the examples of the presentinvention may be used to advantage without the corresponding use ofother features. As such, the foregoing description should be consideredas merely illustrative of the principles, teachings, examples andexemplary embodiments of the present invention, and not in limitationthereof.

It should be understood that these embodiments are only examples of themany advantageous uses of the innovative teachings herein. In general,statements made in the specification of the present application do notnecessarily limit any of the various claimed inventions. Moreover, somestatements may apply to some inventive features but not to others. Ingeneral, unless otherwise indicated, singular elements may be in theplural and vice versa with no loss of generality.

The circuit as described above is part of the design for an integratedcircuit chip. The chip design is created in a graphical computerprogramming language, and stored in a computer storage medium (such as adisk, tape, physical hard drive, or virtual hard drive such as in astorage access network). If the designer does not fabricate chips or thephotolithographic masks used to fabricate chips, the designer transmitsthe resulting design by physical means (e.g., by providing a copy of thestorage medium storing the design) or electronically (e.g., through theInternet) to such entities, directly or indirectly. The stored design isthen converted into the appropriate format (e.g., GDSII) for thefabrication of photolithographic masks, which typically include multiplecopies of the chip design in question that are to be formed on a wafer.The photolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare chip, or in a packaged form. Inthe latter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboard,or other input device, and a central processor.

1. A transistor comprising: a silicon layer including a source regionand a drain region; and a gate stack disposed on the silicon layerbetween the source region and the drain region, the gate stackcomprising a first layer of a high dielectric constant material, asecond layer comprising a metal or metal alloy, and a third layercomprising silicon or polysilicon, wherein a lateral extent of thesecond layer of the gate stack is substantially greater than a lateralextent of the third layer of the gate stack.
 2. The transistor of claim1, further comprising at least one spacer disposed on sidewalls of thegate stack.
 3. The transistor of claim 1, further comprisingsource/drain extensions in the silicon layer.
 4. The transistor of claim1, further comprising: a silicon substrate; and a buried oxide layerover the silicon substrate, wherein the silicon layer covers the buriedoxide layer.
 5. The transistor of claim 1, wherein the first layer ofthe gate stack comprises hafnium dioxide, and the second layer of thegate stack comprises titanium or a titanium alloy.